1. Field of the Invention
This invention relates to the art of continuous separation of magnetic particles from a non-magnetic fluid; more specifically it relates to the continuous separation of such types as they pass through a uniform applied magnetic field; and more specifically it relates to the continuous separation of sub-micron size magnetic particles from viscous flows such as the continuous separation of magnetic catalysts from Fischer-Tropsch wax at operating temperature and pressure or separation of particles of wear from transformer oil or spent engine oil and other non-magnetic hydrophobic or hydrophilic liquids.
2. Description of Related Art
U.S. Pat. No. 4,605,678 describes the application of high gradient magnetic separation technology to separation of iron catalysts employed in Fischer-Tropsch synthesis. This batch operated technology, originally developed for separation of very low concentration and weakly magnetic particles from kaolin clay [R. R. Oder and C. R. Price, “Brightness Beneficiation of Kaolin Clays by Magnetic Treatment,” TAPPI 56, 75 (1973); R. R. Oder, “High Gradient Magnetic Separation: Theory and Applications,” IEEE Transactions on Magnetics, Vol MAG-12, No. 5, pp. 418-425 (September, 1976)] is not well suited to the Fischer-Tropsch application because of the strongly magnetic character of the catalyst particles employed. Additionally, the concentration of these particles in the wax-rich overflow from the reactors employed is so high that the batch process and quasi-continuous versions of it are plugged with the catalyst too rapidly for commercial application. Batch processes, no matter what the nature of the separation mechanism, are not preferred for separating high concentrations of ultra-fine sized particles from high throughput commercial process flows.
U.S. Pat. No. 5,868,939 describes a continuous magnetic separator for breaking emulsions in which a magnetic additive is placed in one phase of the emulsion. The emulsion containing the magnetic additive is made to flow through a vessel containing magnetized rods. As the emulsion flows around the magnetized rods, the magnetic component of the emulsion coalesces and is drawn to the surfaces of the rods by the localized gradient magnetic fields produced by the rods. The magnetic droplets captured by the rods then flow down the surface of the rods to a pool of coalesced material in the bottom of the separator. The two immiscible phases are taken from the separator in separate streams. No means are employed to control the rate of flow of the two streams exiting the coalescer.
WIPO Application No. PCT/US03/02877 describes a continuous magnetic separator for separating magnetic particles from viscous flow in which at least one magnetizable rod is located inside and aligned along the length of a separation chamber to attract magnetic particles from flow around the rods. The rods can be permanent magnets which are magnetized transverse to the rod lengths or can be similarly aligned magnetic rods which are magnetized by an externally applied magnetic field. The magnetic particles are suspended as a slurry in a non-magnetic fluid which enters the chamber between the top where an exit port is located for removing fluid which is low in particle concentration and the bottom of the chamber where an exit port is located for removing a concentrated stream of magnetic particles. The magnetic particles agglomerate by self attraction in the magnetic field surrounding the rods. The agglomerates form chains along the field lines and are attracted to the surfaces of the rods. Fluid flow and gravity drag the chained particles down the rods to the exit port at the bottom of the separation chamber. External means are employed to assure that the greater portion of the mass flow exits the bottom of the separation chamber. The lower ends of the rods and the bottom edge of the magnet generally terminate abruptly at the same elevation in this invention so that there is a large magnetic field gradient at the exit port which tends to hold the magnetic particles inside the separation chamber. This results in a possible buildup of catalyst particles in the bottom of the chamber which can lead to plugging.
U.S. Pat. Nos. 4,605,678 and 5,868,939 and WIPO Application No. PCT/US03/02877 employ the strong field gradients near magnetized surfaces placed in the way of flows containing the magnetic particles to be separated from the flow. While this results in strong magnetic forces for capture, it can also make continuous operation problematical because of the tendency of solid particles to stick and not to release from the magnetized capture surfaces. The invention revealed here overcomes this limitation by use of a separation chamber which contains no magnetic capture elements and which employs means to lessen magnetic forces which would hold the particles in the chamber resulting in plugging.
U.S. Pat. No. 6,068,760, entitled “Catalyst/Wax Separation Device for Slurry Fischer-Tropsch Reactor”, reveals a dynamic settler method whereby micron size iron catalyst particles are separated from Fischer-Tropsch wax by a batch process. This method employs jets of slurry impinging on the bottom of a concentric vessel whereby the catalyst particles are said to settle in the bottom of the vessel for return to the Fischer-Tropsch reactor under gravity flow and particle momentum while a wax of low catalyst concentration flows up through the concentric region of the dynamic settler through a wire mesh filter to down stream upgrading. The patent shows catalyst concentrations achieved in test work versus the upward velocity of flow given in cm/h. It shows that a velocity of 5.9 cm/h is required to make a catalyst concentration in the overflow of the dynamic settler of 0.16% catalyst before wire filtration. With a wax density of nominally 0.8 g/cm3, this corresponds to a filtration rate of nominally 0.8 kg/min/m2. Because of this low rate, many dynamic separators must be employed to handle the output of commercial reactors. It is claimed that the added use of the wire filter permits a speed up of the overflow rate, without revealing what the increase in the overflow rate is, but this happens at the expense of sacrificing the continuous nature of the process. The magnetic method which is disclosed in the present application employs no filters of any kind and is not of the concentric nature of U.S. Pat. No. 6,068,760. Further, the method and apparatus of this invention has achieved filtration rates in continuous throughput which are over 60 times greater than that of the dynamic settler at the same catalyst concentration. The throughput limitation of the dynamic settler is impractical because of the high temperature and pressure employed in the Fischer-Tropsch synthesis. The size and number of dynamic settlers alone would make the method cost prohibitive.